Generally, a liquid crystal display comprises a liquid crystal panel and a backlight unit for irradiating white light onto the back surface of the liquid crystal panel.
In a small or middle-sized (e.g. 20-inch or less) liquid crystal display for use in a personal computer or the like, use is made, as a structure of a backlight unit, of a structure in which a light guide plate is disposed on the back side of a liquid crystal panel and a fluorescent lamp as a light source is disposed on one side or each of both sides of the light guide plate. According to this structure, light from the fluorescent lamp enters the light guide plate from its end face and, while propagating in the light guide plate, part of the light is irradiated toward the back surface of the liquid crystal panel through the front surface of the light guide plate. In this manner, using the light guide plate, the light can be uniformly irradiated onto the back surface of the liquid crystal panel.
The fluorescent lamp used as the light source in the backlight unit of the liquid crystal display is a mercury lamp (a low-pressure mercury vapor discharge lamp, to be exact) with a phosphor coated on the inner surface of a tube thereof. In terms of light emission mechanisms, mercury lamps are classified into the hot cathode type that emits light by thermionic emission and the cold cathode type that emits light by secondary electron emission. The fluorescent lamp of the cold cathode type has a lifetime of as much as about 50,000 hours, which is as much as five times longer than that of the hot cathode type being about 10,000 hours. Accordingly, the fluorescent lamp of the cold cathode type is normally used as a light source for a liquid crystal display.
In the meantime, when a liquid crystal display increases in size, an increase in the quantity of light is also required to a backlight unit. This increase in the quantity of light can be dealt with by increasing the number of fluorescent lamps.
However, in the case of increasing the number of fluorescent lamps, it is necessary to also increase the thickness of a light guide plate (enlarge the incident plane of the light guide plate) according to the number of fluorescent lamps and thus there arises a problem of an increase in weight.
In view of this, in a conventional liquid crystal display, a light guide plate has a V-shaped groove structure on its back side so as to reduce the thickness thereof as approaching a center portion of a screen so that light is efficiently directed toward a liquid crystal panel, thereby achieving a reduction in weight (see, e.g. Patent Document 1).
Further, in the case of increasing the number of fluorescent lamps, there arise problems of an increase in power consumption, an increase in cost due to complication of a circuit structure (addition of an inverter for each fluorescent lamp), and so on. Further, in the case of the fluorescent lamp of the cold cathode type, even if the tube diameter is enlarged to increase the light emission amount per lamp, the light emission efficiency is lowered in inverse proportion to the tube diameter and thus there is also a problem of an increase in power consumption.
In view of this, in another conventional liquid crystal display, for the purpose of a reduction in power consumption and so on, the hot cathode type is used, instead of the cold cathode type, as a fluorescent lamp for use in a backlight unit (see, e.g. Patent Document 2).
The fluorescent lamp of the hot cathode type can achieve a light emission efficiency as high as twice or more that of the fluorescent lamp of the cold cathode type and, further, even if the quantity of light is increased by enlarging the tube diameter, the light emission efficiency is not lowered. Therefore, in illuminators and so on, use has already been made of fluorescent lamps of the hot cathode type with a diameter of about 30 mm that can achieve a light emission amount (total luminous flux) of as much as 2000 lumens or more per lamp (a quantity of light as much as about ten times that of a cold cathode tube with a diameter of 2 to 3 mm).    Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2001-228477 (particularly, Abstract and Paragraph 0005)    Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. 2000-187211 (particularly, Paragraph 0003)